Magnetic sensors can be used in many applications including magnetometry, magnetic sensing, magnetic imaging, and magnetic testing. Possible field applications include navigation, magnetic anomaly detection, magnetic compassing, and magnetic mapping.
The invention of the scanning tunneling microscope has stimulated new ideas for nanoscale technology, including new types of sensors that exploit properties of tunneling transducers. A particularly important property that distinguishes tunneling transducers from other transducers is that tunneling transducers have inherent gain. The gain is a consequence of the exponential dependence of the tunneling current on electrode spacing. Because of the gain, the signal from a tunneling transducer is much larger than from a conventional transducer with the result that much less amplification is needed to get a useable signal. Consequently, the noise added in the amplification stage is greatly reduced. Electron tunneling devices also have the advantage that they can be scaled down without losing sensitivity.
The tunneling transducers are characterized by a tip electrode that is spaced from a counter electrode by about 1 nm. The tunneling gap is the space between the tunneling tip and the counter electrode. The counter electrode and the gap configuration constitute a tunneling junction.
For a typical tunneling sensor, the current increases by a factor of ten for each 0.1 nm change in the tunneling gap. In the most common configuration, the tunneling current, or the current in the gap between the tip and the counter electrode, is maintained constant by the use of a feedback control circuit and a displacement actuator that adjusts the width of the gap to maintain the constant current. The feedback control circuit is used to compare the tunnel current to a reference value and to generate an error signal that is proportional to the difference. The error signal is used to control the displacement actuator that, in turn, controls the size of the tunneling gap.
U.S. Pat. No. 5,103,174 discloses the use of scanning tunneling technology in sensing devices for measuring the effect of magnetic field variations on magnetostrictive materials. Magnetostrictive materials are characterized by dimension changes in response to changes in the local magnetic field.
Table 1, below, is a listing of the properties of several magnetic field sensors, including the sensor described herein. The sensitivity listed for the tunneling sensor is based on the noise limit set by Brownian motion of a suspended magnet.
TABLE 1 ______________________________________ Sensitivty Volume Power Type (nTesla/.sqroot.Hz) (cm.sup.3) (mW) ______________________________________ Squid (at 4 K) 10.sup.-6 10 1000 Fiber Optic 7 .times. 10.sup.-5 60 500 Optically Pumped 10.sup.-3 500 7500 Tunneling 10.sup.-3 10 1 Proton Precession 10.sup.-2 500 1000 Fluxgate 3 .times. 1O.sup.-2 50 500 Suspended Magnet 10.sup.-1 200 40 Magnetotransistor 10 1 &lt; 1 Hall Probe 1000 5 100 Magnetodiode 1000 1 &lt; 1 Magnetoresistor 1000 1 &lt; 1 ______________________________________